Field of the Invention
The present invention relates to an NMR sample tube and NMR spectrometer.
Description of Related Art
An NMR (nuclear magnetic resonance) spectrometer is an analytical instrument for detecting a signal arising from atomic nuclei having spin magnetic moments by applying a static magnetic field to the nuclei to induce the spin magnetic moments for producing a Larmor precession and irradiating the nuclei with RF waves having the same frequency as the precession to bring the nuclei into resonance.
Samples to be investigated by NMR include two types: solution samples and solid samples. Among them, many solution samples give quite sharp NMR spectra and, therefore, it is widespread to perform molecular structural analysis of chemical substances from the obtained high-resolution NMR spectra.
On the other hand, in an NMR spectrum of a sample in solid phase, interactions (such as dipolar interactions) which would be nullified by rotational Brownian motion in a solution manifest themselves directly and so the spectral linewidth broadens extremely, thus obscuring chemical shift terms. Therefore, in an NMR spectrum, it is impossible to isolate the signal peaks arising from various portions of a molecule under investigation. As a result, it has been thought that solid-state NMR spectroscopy is unsuited for molecular structural analysis.
MAS (magic angle spinning) has attracted attention as a method of overcoming this undesirable phenomenon and giving rise to sharp solid-state NMR spectra. In particular, anisotropic interactions are removed and chemical shift terms can be extracted by tilting the sample tube at the magic angle of 54.7° to the static magnetic field and spinning the tube at high speed.
For example, JP-A-2011-227036 discloses an NMR spectrometer equipped with a sample spinner having a gas bearing that supplies gas into between a sample tube and a sample tube-holding mechanism to keep the sample tube afloat. The gas bearing permits the sample tube to be spun at high speed about an axis tilted at 54.7° to the static magnetic field.
In such an NMR spectrometer, it is generally required that the sample tube be spun at high speeds of several kilohertz to tens of kilohertz within the static magnetic field in order to perform good NMR spectroscopy of solid samples employing MAS.
To implement the MAS method, a solid sample placed within a static magnetic field must be spun at high speed. However, it is not easy to obtain rotational speeds of kilohertz to tens of kilohertz which are regarded as needed spinning speed. Accordingly, gas bearing techniques have been heretofore adopted to obtain such rotational speeds, and various methods have been proposed.
FIG. 11 shows a conventional high-speed spinner for solid-state NMR spectroscopy. The spinner includes a cylindrical stator 11 that surrounds a rotor 12 with a slight spacing therebetween. A solid sample is sealed in the rotor 12. The bottom of the cylindrical stator 11 is covered by a thrust stator 13. A thrust rotor 14 is mounted at the bottom of the rotor 12 in an opposite relation to the thrust stator 13 to maintain the position of the rotor 12 taken in the thrust direction. A turbine 16 is mounted in an upper part of the rotor 12 to impart a rotating force to the rotor 12 by gas jets ejected from turbine nozzles 15 formed in the stator 11. The rotor 12, thrust rotor 14, and turbine 16 together constitute a rotor-turbine assembly that spins at high speed.
FIG. 12 is a cross section of a conventional high-speed spinner for solid-state NMR spectroscopy, the cross section being taken along line bb of FIG. 11. As is obvious from FIG. 12, a thin layer of gas is formed between the stator 11 and the rotor 12 by continuously supplying gas from plural gas feeding holes 911 formed in the stator 11 toward the interior of the stator 11. This results in a journal gas bearing. That is, the frictional resistance between the stator 11 and the rotor 12 is reduced to a minimum. Consequently, the rotor-turbine assembly can be spun inside the stator 11 at high speed.
FIG. 13 is a cross section of the conventional high-speed spinner for solid-state NMR spectroscopy, the cross section being taken along line cc of FIG. 11. As is obvious from FIG. 13, gas jets ejected from the turbine nozzles 15 formed eccentrically relative to the stator 11 act on the blades of the turbine 16, imparting a rotating force on the rotor-turbine assembly. The gas jets acting on the turbine 16 change in orientation and form gas streams 17 shown in FIG. 11, the streams 17 being discharged out of the high-speed spinner.
Development of a high-speed spinner using such a hydrostatic bearing was commenced by Doty (U.S. Pat. No. 4,456,882). Then, Bartuska et al. (U.S. Pat. No. 4,511,841) have proposed a high-speed spinner using a combination of a hydrostatic bearing and a hydrodynamic bearing. Doty et al. (U.S. Pat. No. 5,508,615) have attempted to make improvements in the hydrostatic bearing.
It has been found that when the above-described high-speed spinner is used and the spinning rate of the sample tube is raised, if the natural vibration frequency of the sample tube comes into coincidence with the spinning speed, synchronous vibrations occur. This phenomenon is observed when a rotor-turbine assembly supported by a gas bearing is spun at high speed. Generally, the phenomenon is caused by an imbalance in the rotor-turbine assembly. If the imbalance is large, the sample tube may come into contact with the gas bearing at the resonant point that is a natural vibration frequency at which synchronous vibrations take place, thus causing seizure or damage. This makes it impossible to raise the spinning speed. This will be described in further detail below.
The resonant point of synchronous vibrations is given as follows.
In cylindrical mode, the resonant point is given by
      N    1    =            1              2        ⁢        π              ⁢                  (                  k          M                )                    1        2            
In conical mode, the resonant point is given by
      N    2    =            1              2        ⁢        π              ⁢                  (                              2            ⁢            k            ⁢                                                  ⁢                          J              2                                                          I              t                        -                          I              0                                      )                    1        2            where M is the mass of a rotor-turbine assembly supported by one radial bearing, k is the bearing rigidity per radial bearing, It is the inertial moment about the center of gravity of the rotor-turbine assembly, I0 is the polar inertial moment about the central line of the rotor-turbine assembly, and J is a half of the center-to-center distance of two radial bearings.
FIG. 14 is a graph showing the rotational characteristics of a rotor-turbine assembly supported by a gas bearing. The horizontal axis indicates the spinning rate of the rotor-turbine assembly. The vertical axis indicates the amplitude of swings of the rotor-turbine assembly. As shown in FIG. 14, in order to spin the rotor-turbine assembly at high speed, the two resonant points given by the above-described equations must be exceeded. It is necessary to reduce the imbalance in the rotor-turbine assembly to permit it to be spun at high speeds beyond the resonant points.
Therefore, in order to spin the sample tube at high speed in an NMR instrument, the sample tube is required to be filled up with a sample such that the imbalance in the filled sample tube is reduced. However, solid samples filling the sample tube may assume various states such as powdered state, pasty state, and rubber-like state. Also, solid samples assume various forms such as particulate form, filmy form, and block form. For these reasons, it is difficult to fill up a sample tube with a sample with a small amount of imbalance. It has been difficult to spin a sample tube at high speed stably.